According to the BIOS release notes, nothing major has been done to the BIOS since 2012.

Based upon this, I would be willing to bet that the board's Secure Boot feature has never been tested with Windows 10.

Bottom line, if it didn't work with the DH61DL -- and you were doing it properly -- it's unlikely that it will work with the DQ67EP. Further, since both of these boards are in the end-of-life and end-of-interactive-support states -- and Intel has exited the Desktop Boards business altogether -- if the feature is broken, there is absolutely zero chance of it being fixed (there's simply no one left to do it).

Well I feel like an idiot. On the DH61DL boards I've tested I enabled the SecureBoot but it always showed is as Off in msinfo32. But looking at the Secure Boot settings in the BIOS, everything was installed except the Intel Platform Key (PKpub). I assumed that was something that had to be loaded from a file from the PC builder so I never tried installing it. Turns out it doesn't require anything; I just had to change "Install Intel Platform Key" to Enabled, save the BIOS & reboot, and now PKpub says "Installed". Once this is done SecureBoot shows it's turned On in Windows 10.

I'm betting the DQ67EP is probably the same way. I don't have access to the one we have right now to test it.